The present invention relates to an optical encoder for generating output pulses in number proportional to the amount of preselected movement of a coupled analog component.
An encoder is an electromechanical device to measure the position, velocity, acceleration, etc. of an operating mechanism to which the encoder is coupled. Optical encoders utilize light as a means for transmitting information about mechanical movement to a detector which outputs a number of electrical pulses dependent on the amount of movement of the operating mechanism.
Encoders may be linear or rotary. Both utilize a moving unit having alternating strips of transparency and opacity to the light path. The size, shape and frequency of these areas determines the number and rate of the output pulses for a given movement of the operating mechanism. A common moving unit is in the form of a disc having a plurality of short radially directed spaced apart openings around its periphery. A light emitting diode (LED) directs light onto the disc and a photosensor located in the opposite side of the disc detects transmitted light giving an output proportional to the intensity of light transmitted through each open region. Such an encoder in combination with a counter for storing the number of sensor output pulses measures only the magnitude of movement, the speed, etc. but not the direction. Moreover, such discs provide a resolution of less than 50 lines per inch around the disc periphery. Greater resolution is possible using chrome on glass but at a greatly increased cost.
Mylar film can also be used to obtain higher resolution but offers poor mechanical, thermal and humidity stability and is easily damaged by handling.
Resolution may be improved by using a reticle having a pattern of transparent and opaque areas which are optically mated to those patterns on the moving unit. As the moving unit moves relative to the reticle which is stationary, alternating alignment and non-alignment of the transparent regions causes the transmitted light intensity to rise and fall. If the size of the transparent and opaque regions are equal then the degree of fluctuation of the transmitted light will be maximized.
In order to determine direction of movement of the moving unit one can locate two LED's and two corresponding sensors with one sensor detecting light transmitted through a first reticle opening and a second sensor detecting light transmitted through a second reticle opening. By shifting the second reticle opening such that it is 1/4 cycle from the pattern in the first reticle a phase shift in the output from the second detector relative to the first is achieved. This phase shift reverses when the direction of the moving unit reverses.
There has been a severe problem in attempting to make high resolution encoders capable of quadrature at a reasonable cost. This difficulty arises because of many requirements. First, the spacing between the moving unit and reticle must be as small as possible and should not vary. The thickness of the moving unit and reticle must be small enough to minimize diffraction along the edges and yet be thick enough to cut down light transmission significantly. The tolerance in spacing of the transparent and opaque regions must be low. The material used must be stable against changes in humidity, have a low temperature coefficient of expansion and be sufficiently rugged to withstand handling. Hitherto, rotary encoders having a large number of lines per inch (400 or more) meeting the above requirements have only been built with great difficulty and usually individual adjustment.
Optical encoders have been utilized in track balls which consist of a large steel ball rotatably mounted on a casing. Clutch discs contact the ball at orthogonal positions relative to the center of the ball and couple to a pair of respective rotary encoders which employ a disc. Because of the use of the disc the height of the casing is necessarily relatively large to accommodate the diameter of the disc. Moreover, only a simple encoder is used. A similar encoder is used in a "mouse" which is a casing having a large ball which is adapted to roll over a table top. To date these have been the only known units for generating input control data for a computer using optical encoders.
Accordingly, it is an object of the present invention to provide an improved low cost optical encoder.
It is a further object of the invention to provide an encoder of substantially higher resolution than those used in present computer analog input devices and capable of quadrature.
It is yet a further object of the invention to provide an improved joystick which has a low profile and capable of high resolution and able to generate information as to motion and direction of motion.